Nanoparticles mean particles having the size of approximately 1 nm to 100 nm. For past decades, various nanoparticles have been synthesized since nanoparticles have unique characteristics which other particles bigger than micron-size do not have, due to the quantum size effect, as well as nanoparticles have excellent characteristics such as luminescence characteristics, magnetic properties and catalytic characteristics due to the relatively huge surface area even in case of having same characteristics.
Particularly among the above nanoparticles, magnetic nanoparticles have excellent magnetic properties, and when the particle sizes are less than approximately 20 nm, each nanoparticle shows super paramagnetism, which has been drawing attention. Super paramagnetic nanoparticles are thought to be ideal to be applied in various fields, since super paramagnetic nanoparticles usually disperse evenly without showing general magnetic properties and the gap between the nanoparticles can be adjusted by external magnetic fields.
For the magnetic nanoparticles to be applied effectively, such particles should retain excellent magnetic properties, stably disperse in various environments and be able to easily combine with functional material.
Generally, magnetic nanoparticles have been synthesized by a co-precipitation process synthesizing 2+ or 3+ metal salts in alkaline aqueous solution at approximately 100° C. and the synthesized nanoparticles are called ferrofluid in occasion since the particles have excellent dispersion characteristics in aqueous solutions (U.S. Pat. No. 4,019,994; J. R. Kelley ‘Process for the preparation of aqueous magnetic material suspensions’, J. Mol. Liq., 1999, 83, 217; R. Massart et al. ‘New aqueous magnetic fluids’, J. Phys. Chem. B, 2001, 105, 1168; J. Depeyrot et al. ‘New electric double-layered magnetic fluids based on copper, nickel, and zinc ferrite nanostructures’). However, there are problems with the magnetic nanoparticles prepared by the co-precipitation process such as irregular size distribution and difficulties in adjusting the particle size, as well as low crystalline property due to the low synthetic temperature which leads to weaker magnetic properties.
To solve this problem, methods to synthesize nanoparticles with excellent magnetic properties and regular size have been developed and the best known method is pyrolysis of metal salt or complex in organic solution (U.S. Pat. No. 6,676,729 B2; S. Sun ‘Metal salt reduction to form alloy nanoparticles’, U.S. Pat. No. 7,407,527 B2; T. Hyeon ‘Synthesis of mono-disperse and highly crystalline nano-particles of metals, alloys, metal-oxides, and multi-metal oxides without a size-selection process’). However, such magnetic nanoparticles synthesized by pyrolysis are limited to be applied in bio-fields, since there are difficulties in surface treatments, physical characteristics are inhibited by the organic ligands existing on the surface, and the particles do not disperse in aqueous solution due to the long alkyl chain organic molecule ligands on the surfaces of nanoparticles since the nanoparticles are protected or strongly combined with long alkyl chain organic molecule ligands on the surfaces.
To solve the above problem, methods have been developed to form nanocluster colloid by spontaneous agglomeration along with the reaction progress, instead of combining each nanoparticle using organic/inorganic material (Angew. Chem. Int. Ed., 2005, 44, 2782; Y. Li et al., ‘Monodisperse magnetic single-crystal ferrite microspheres’, J. Am. Chem. Soc., 2009, 131, 12900; A. Gupta et al., ‘Adjusted growth of monodisperse self-supported superparamagnetic nanostructures of spherical and rod-like CoFe2O4 nanocrystals’).
However, the above synthetic method does not allow mass production which is essential for practice or industrial use. Although magnetic nanocluster colloid prepared by the above method has excellent magnetic properties and dispersion characteristics, there are problems in mass production since its synthetic condition is complex and dangerous and the reaction generally occurs in a high-pressure reactor.